The SV40 large tumor antigen (TAg) transforms a variety of cell lines and induces tumors in rodents. In addition, TAg is required to replicate of the SV40 genome, assemble infectious virions and activate the transcription of the SV40 capsid protein genes. While it is clear that these activites require TAg to interact with multiple cellular targets, the mechanism of TAg action and the effect of TAg on these factors is unknown. However, the presence of a functional DnaJ-domain in the amino terminus of TAg suggests that TAg participates with other cellular chaperones to mediate changes in multi-protein complexes involved in these processes. To investigate the chaperone activity of TAg, novel mutations in the J-domain of TAg will be isolated using our recently developed genetic screen in yeast. These mutations will provide insight into the structure-function relationship of the J-domain and will permit further investigation of the role of TAg chaperone activity in SV40 infection and tumorigenesis. Mutations in TAg that compromise its chaperone activity in yeast will be tested in vitro for the ability to stimulate hsp70 and in vivo for the capability to assemble infectious virions, replicate SV40 DNA and transform established cell lines. These experiments will test the "Chaperone Model" for TAg activity by determining whether a functional interaction between TAg and cytosolic hsp7O is required for SV40 infection. The results from these experiments will influence our understanding of virally-induced tumorigenesis and the mechanisms of molecular chaperones in general.